This invention relates to a tomographic method for a body, utilizing nuclear magnetic resonance phenomena, and in particular to a method for imaging blood flow in a blood vessel of a body.
A prior art representative method for imaging blood flow is discussed in JP-A-61-88132 (U.S. patent application Ser. No. 763096) or Radiology, May (1986) pp. 411-418. According to this method, using a sequence giving some influences on blood flow parts in the intensity without giving any influence on static parts, two images are scanned and blood vessel parts are extracted by using a differential image thereof so that the intensity varies only in the blood vessel parts. This principle utilizes the fact that the phase of the spin in the blood vessel varies proportionally to the speed of the blood movement and as the result the intensity varies. That is, at the imaging, when the intensity is measured in synchronism with signals coming from an electrocardiograph, the flow speed is different for the diastolic period and the systolic period of the heart. Consequently the static parts are imaged with a constant intensity and the parts including blood vessel are imaged with different intensities, when they are imaged according to the sequence described previously. Therefore only the blood vessel parts are extracted by forming the difference between these two images.
There is known another method, by which, as a measure for obtaining variations in the intensity for the blood vessel parts, instead of imaging them at two points of time, at which the flow speed is different, they are imaged according to two different sequences so that the gain in the phase variation due to the speed is different therefor at which the flow speed is the same, but its fundamental principle is the same in that variations in the phase of the spin depending on the speed is utilized.
According to the prior art techniques, in practical use, there is a problem to decide with what the imaging should be synchronized. Usually, as indicated in FIG. 1A, the R wave 100 in the electrocardiogram is detected and the imaging is synchronized therewith with a delay of dt so as to be in accordance with the variations in the flow speed. If the delay time from the R wave is not appropriately set, the difference between the two images is not significant and therefore it is not possible to obtain a precise blood vessel image. In order to overcome this inconvenience, heretofore, the imaging was effected several times e.g. with an interval of 50 m sec as the delay time, followed by an image selection process. Since usually an imaging time of about 4 to 8 minutes is necessary for one image, this method has a problem that it takes too much time to obtain one blood vessel image.
Alternatively there is another technique for measuring the flow speed by using an ultrasonic doppler apparatus as a different flow speed measuring apparatus. However it requires apparatuses other than MRI (Magnetic Resonance Imaging) apparatus. Therefore, there is a problem that it requires much time and that a region to be examined is restricted due to the doppler measuring apparatus.